The long term objectives of this work are to contribute to the elucidation of the normal structural organization and development of the vertebrate visual system, and to assess the changes that occur when the system is deprived of normal visual input. The proposed studies are designed to clarify the contributions of specific factors that are likely to play a key role in the establishment of the set of axonal connections characteristic of the mature, vertebrate visual system. In the coming grant period, we propose to examine the following: (1) The development of topographic order in the avian retinotectal projection, with an emphasis on the occurrence of targeting errors, and the abilities of retinal axons that make such errors to respond to positional cues that have been postulated to guide the establishment of topographic order. The formation and fate of aberrant axon branches and arbors will be studied in normal tectum, and in tectal regions deprived of its normal retinal input, to assess the role of positional markers in the process of map refinement. (2) A clarification of the role of a substantial, transient retinofugal projection that appears to act as a "template" to guide outgrowing axons of the isthmo-optic nucleus, the source of centrifugal innervation of the avian retina, to the target, the contralateral retina, will be undertaken. (3) The establishment of topographic order in the mammalian retinocollicular projection will be studied in normal rats using high resolution anterograde axon tracers. This will serve as a baseline study for an analysis of the role of neural activity in the refinement of the precision of the topographic map between the retina and colliculus. (4) The mechanisms of axon collateral formation and elimination of subcortical projections of the primary visual cortex will be addressed in rats. (5) A heterochronic, homotopic transplant paradigm will be used to test whether "waiting periods", the phenomenon where axons reach their target but pause for several days prior to extending into it, is related to target immaturity, or may be a feature of axon growth programs.